feat: support grouping by expression

Closes #329 (closed)

Now expressions can be used in group by: select a + b from t group by a + b

Note:

• select a + b as q from t group by q - is not supported and is not part of this mr
• in some expression user must enclose the grouping expression in projection: select (a+b)+count(c) from t group by a + b will work, but select a + b + count(c) from t group by a + b will not work